Active material for battery and method of preparing the same

ABSTRACT

An active material for a battery has a surface treatment layer that includes a conductive agent and at least one coating-element-containing compound selected from the group consisting of a coating-element-containing hydroxide, a coating-element-containing oxyhydroxide, a coating-element-containing oxycarbonate, a coating-element-containing hydroxycarbonate, and a mixture thereof.

CROSS REFERENCE TO RELATED APPLICATION

This application is divisional application of U.S. patent applicationSer. No. 10/189,384, now U.S. Pat. No. 7,108,944, filed Jul. 8, 2002 andbased on Korea Patent Application No. 2001-43554 filed on Jul. 19, 2001in the Korean Intellectual Property Office, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active material for a battery and amethod of preparing the same, and more specifically to an activematerial for a battery with excellent electrochemical characteristicsand thermal stability, and a method of preparing the same.

2. Description of the Related Art

Due to recent trends toward more compact and lighter portable electronicequipment, there has been a growing need to develop a high performanceand large capacity battery to power this portable electronic equipment.In particular, there has been extensive research to provide suchbatteries with good safety characteristics and having a low cost.

Generally, batteries are classified as primary batteries, which are usedonly once before being discarded, and secondary batteries, which arerechargeable for multiple uses. The primary batteries include manganesebatteries, alkaline batteries, mercury batteries, silver oxide batteriesand so on. The secondary batteries include lead-acid storage batteries,Ni—MH (nickel metal hydride) batteries, nickel-cadmium batteries,lithium metal batteries, lithium ion batteries, lithium polymerbatteries, lithium-sulfur batteries and so on.

Lithium ion secondary batteries use materials that reversiblyintercalate or deintercalate lithium ions during charge and dischargereactions for both positive and negative active materials, and containan organic electrolyte or polymer electrolyte between a positiveelectrode and a negative electrode having the positive and negativeactive materials, respectively. These batteries generate electricalenergy due to changes in chemical potential during theintercalation/deintercalation of the lithium ions at the positive andnegative electrodes.

Factors that affect a battery's performance characteristics, such ascapacity, cycle life, power capability, safety, and reliability, includeelectrochemical properties and thermal stability of the active materialsthat participate in the electrochemical reactions at the positive andnegative electrodes. Therefore, there are continuing research efforts tofind improvements in the electrochemical properties and thermalstability of the active materials at the positive and negativeelectrodes.

Of the active materials which have been considered for the activematerial of the negative electrodes, lithium metal gives both a highcell capacity and a high voltage because the lithium metal has a highelectrical capacity per unit mass and a relatively highelectronegativity. However, since it is difficult to assure the safetyof the battery using lithium metal, a carbonaceous material that is ableto intercalate and deintercalate lithium ions is used extensively forthe active material of the negative electrodes in lithium secondarybatteries. With the use of the carbonaceous material, the batteryperformance, especially, cycle life and safety, has improvedtremendously from that of the lithium metal battery. In order to furtherimprove the negative electrode performance, it has been suggested to addan additive, such as boron, to the carbonaceous material, especially bycoating with the additive. For example, a boron-coated graphite (BOC)improves the performance characteristics of the carbonaceous materials.

Lithium metal compounds of a complex formula are often used as apositive active material of the lithium secondary battery. Typicalexamples include LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1-x)CO_(x)O₂(0<x<1),LiMnO₂ and a mixture of these compounds. Manganese-based positive activematerials such as LiMn₂O₄ or LiMnO₂ are relatively easy to synthesize,less costly than the other materials, and environmentally friendly.However, these manganese-based materials have a disadvantage in having arelatively low capacity. On the other hand, LiCoO₂ has many technicaladvantages over the other materials such as relatively good cycle lifeand relatively high specific energy. This compound is presently the mostpopular material for positive electrodes of commercially availableLi-ion batteries. However, it is relatively expensive. While it isdesirable to further improve its stability on charge-discharge cyclingat a high rate, it is one of the most stable compounds of the presentlyavailable positive active materials. LiNiO₂ has the highest dischargecapacity of all positive active materials mentioned above, but it isdifficult to synthesize and is the least stable among the compoundsmentioned above.

Among these compounds, LiCoO₂ is the most well accepted in the batteryindustry since its overall performance characteristics, especially,cycle life, are superior to the others. Accordingly, most of thecommercially available rechargeable lithium batteries adopt LiCoO₂ asthe positive active material, although its cost is relatively high.There is a great deal of research effort in the industry to develop afurther improved active material in overall performance as well as toreduce the cost, if possible.

One of the previous efforts includes substituting a part of theexpensive Co from LiCoO₂ with other less expensive metals. For instance,SONY CORPORATION prepared Li_(x)Co_(1-y)M_(y)O₂ by doping about 1 to 5percent by weight of Al₂O₃ into LiCoO₂. A&TB (ASAHI & TOSHIBA BATTERYCO.) prepared an Sn-doped Co-based active material by substituting apart of the Co from LiCoO₂ with the Sn.

Another approach is to coat a lithiated compound with a coatingmaterial. In U.S. Pat. No. 5,292,601, Li_(x)MO₂ (where M is at least oneelement selected from Co, Ni, and Mn; and x is 0.5 to 1) is suggested asan improved alternative material over LiCoO₂. U.S. Pat. No. 5,705,291suggests a method in which a composition comprising borate, aluminate,silicate, or mixtures thereof is coated onto the surface of a lithiatedintercalation compound.

Japanese Patent Laid-Open No. Hei 9-55210 discloses coating a lithiumnickel-based oxide with an alkoxide of Co, Al and Mn, and performing aheat-treatment to prepare a coated positive active material. JapanesePatent Laid-Open No. Hei 11-16566 discloses coating a lithium-metaloxide with another metal and/or an oxide thereof. The another metalincludes Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo. Japanese PatentLaid-Open No. Hei 11-185758 discloses coating the surface of a lithiummanganese oxide with a metal oxide by using a co-precipitation processfollowed by heat-treating the same to prepare a positive activematerial.

In the above description, positive active materials of lithium secondarybatteries and related examples of developments were explained. Recently,with demands for more compact and light weight portable electronicequipment, various types of batteries including a Li-ion battery havesimilar demands for an improved active material that can assure goodbattery performance, safety, and reliability. A great deal of theresearch and development efforts have been devoted to improvements onperformance and thermal stability of the positive active materials toensure improved cell performance, safety, and reliability of batteriesunder various use conditions, including many abuse conditions.

SUMMARY OF THE INVENTION

In order to solve the above and other problems, it is an object of thepresent invention to provide an active material for a battery with goodelectrochemical characteristics, such as capacity, cycle life, dischargepotential, power capability, and other similar electrochemicalcharacteristics.

It is another object to provide a method of preparing an active materialwith good manufacturing productivity and having an economicalpreparation process.

It is still another object to provide an electrode with a high energydensity.

It is a further object to provide a battery with high batteryperformance and safety.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part, will be obvious fromthe description, or may be learned by practice of the invention.

In order to accomplish these and other objects, an embodiment of thepresent invention provides an active material for a battery, the activematerial having a surface treatment layer comprising a conductive agentand at least one compound selected from the group consisting of acoating-element-containing hydroxide, a coating-element-containingoxyhydroxide, a coating-element-containing oxycarbonate, acoating-element-containing hydroxycarbonate, and a mixture thereof.

According to another embodiment of present invention, a process ofpreparing an active material for a battery includes adding a conductiveagent and a coating-element source to a solvent selected from the groupconsisting of water, organic solvent, and mixtures thereof to prepare acoating liquid, adding the active material to the coating liquid to coatthe active material, and drying the coated active material to form asurface-treatment layer comprising the conductive agent and at least onecompound selected from the group consisting of acoating-element-containing hydroxide, a coating-element-containingoxyhydroxide, a coating-element-containing oxycarbonate, acoating-element-containing hydroxycarbonate, and a mixture thereof.

According to a further embodiment of the present invention, an electrodewith a high energy density comprises an active material coated with acoating comprising a conductive agent and at least one compound selectedfrom the group consisting of a coating-element-containing hydroxide, acoating-element-containing oxyhydroxide, a coating-element-containingoxycarbonate, a coating-element-containing hydroxycarbonate, and amixture thereof.

According to yet another embodiment of the present invention, a processof preparing an electrode with a high energy density for a batteryincludes adding a binder material to a solvent to prepare abinder-containing solution, adding an active material coated with acoating including conductive agent and at least one compound selectedfrom the group consisting of a coating-element-containing hydroxide, acoating-element-containing oxyhydroxide, a coating-element-containingoxycarbonate, a coating-element-containing hydroxycarbonate, and amixture thereof to the binder-containing solution, to prepare an activematerial slurry, and casting the active material slurry on a currentcollector to fabricate an electrode for a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will become more readily apparent and more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when considered in conjunction with theaccompanying drawings, wherein:

FIGS. 1A and 1B are schematic diagrams illustrating production processesof an active material according to a general process and an embodimentof the present invention (one-shot coating process);

FIG. 2 is a schematic diagram showing an apparatus used in a coatingprocess according to another embodiment of the present invention;

FIGS. 3A and 3B are schematic diagrams illustrating production processesof an electrode according to a conventional process and a furtherembodiment of the present invention;

FIG. 4A is a schematic illustration showing the distribution of anactive material and a conductive agent in an electrode preparedaccording to a conventional process;

FIG. 4B is a schematic illustration showing an active material preparedaccording to an embodiment of the present invention;

FIGS. 5A and 5B are Scanning Electron Microscopy (SEM) photographs of apositive active material powder in the positive electrodes according toComparative Example 1 and Example 1 of an embodiment of the presentinvention, respectively;

FIGS. 6A and 6B are graphs showing the capacity and the voltageperformances at rates of 0.2C, 0.5C, and 1C for battery cells accordingto Reference Example 5 and Example 5 of an embodiment of the presentinvention, respectively;

FIG. 7 is a graph showing charge-discharge characteristics for batterycells at 1C rate according to Comparative Example 5, Reference Example4, and Example 3 of an embodiment of the present invention;

FIG. 8 is a cross-sectional view of a prismatic Li-ion cell according toan embodiment of the invention;

FIG. 9 is a graph showing cycle life characteristics at 1C rate for theprismatic Li-ion cells comprising active materials prepared inComparative Example 5 and Example 3 of an embodiment of the presentinvention;

FIG. 10 is a graph showing discharge characteristics at a lowtemperature of −20° C. at 0.2C rate for the prismatic Li-ion cellscomprising active materials prepared in Reference Example 5 and Example5 of an embodiment of the present invention; and

FIG. 11 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained from charged (4.3 V) prismatic Li-ion cellscomprising active materials prepared in Comparative Example 5 andExample 5 of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings and discussed in relation to specificExamples, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures and the specific Examples.

The active material for a battery of the present invention is coatedwith a surface-treatment layer comprising a conductive agent and atleast one compound (hereinafter referred to as a“coating-element-containing compound”) selected from the groupconsisting of a coating-element-containing hydroxide, acoating-element-containing oxyhydroxide, a coating-element-containingoxycarbonate, a coating-element-containing hydroxycarbonate, and amixture thereof.

The surface treatment (coating) technique of the active material withthe conductive agent and the coating-element-containing compound of thepresent invention may be used for many different types of batteries, andis effective in improving the performance characteristics of activematerials for both the positive electrodes and the negative electrodes.

The active material for the surface treatment includes materials thatcan undergo reversible electrochemical oxidation-reduction reactions.The reversibly oxidizable and reducible materials include a metal, alithium-containing alloy, sulfur-based compounds, elements or compoundsthat can reversibly form lithium-containing compounds by a reaction withlithium ions, and all materials that can reversiblyintercalate/deintercalate lithium ions (lithiated intercalationcompounds), although the present invention is not limited thereto.

According to embodiments of the invention, the metal includes lithium,tin, or titanium, and the lithium-containing alloy includes an alloycomprising lithium and a metal selected from the group consisting of Na,K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn. Preferably, thelithium-containing alloy is a lithium/aluminum alloy, a lithium/tinalloy, or a lithium/magnesium alloy. The sulfur-based compound which isthe positive active material of the lithium-sulfur battery includes asulfur element, Li₂S_(n)(n≧1), an organosulfur compound, and acarbon-sulfur polymer ((C₂S_(x))_(n) where x=2.5 to 50 and n≧2). Theelements or compounds which can reversibly form a lithium-containingcompound by a reaction with lithium ions include silicon, tin oxide(SnO₂) and titanium nitrate.

The active material that reversibly intercalates/deintercalates lithiumions (such as lithiated intercalation compounds) includes carbon-basedmaterials, lithium-containing metal oxides, and lithium-containingchalcogenide compounds. The carbon-based material can be amorphouscarbon, crystalline carbon, or a mixture thereof. Examples of theamorphous carbon include soft carbon (low temperature calcinatedcarbon), and hard carbon (high temperature calcinated carbon). Examplesof crystalline carbon include natural graphite or artificial graphitewhich are plate, sphere, or fiber shaped.

A conventional lithium-containing compound (lithium-containing metaloxide and lithium-containing chalcogenide compound) may be used as thelithiated intercalation compound of the present invention. Specificexamples are represented in the following formulas (1) to (13):Li_(x)Mn_(1-y)M_(y)A₂  (1)Li_(x)Mn_(1−y)M_(y)O_(2-z)X₂  (2)Li_(x)Mn₂O_(4-z)X_(z)  (3)Li_(x)Mn_(2-y)M_(y)A₄  (4)Li_(x)Co_(1-y)M_(y)A₂  (5)Li_(x)Co_(1-y)M_(y)O_(2-z)X_(z)  (6)Li_(x)Ni_(1-y)M_(y)A₂  (7)Li_(x)Ni_(1-y)M_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (9)Li_(x)Ni_(1-y-z)Co_(y)M_(z)A_(a)  (10)Li_(x)Ni_(1-y-z)Co_(y)M_(z)O_(2-a)X_(a)  (11)Li_(x)Ni_(1-y-z)Mn_(y)M_(z)A_(a)  (12)Li_(x)Ni_(1-y-z)Mn_(y)M_(z)O_(2-a)X_(a)  (13)

wherein

0.95≦x≦1.1; 0≦y≦0.5; 0≦z≦z≦0.5; 0≦a≦2;

-   -   M is at least one element selected from the group consisting of        Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and rare earth elements;    -   A is at least one element selected from the group consisting of        O, F, S, and P; and    -   X is at least one element selected from the group consisting of        F, S, and P.

The average particle size of these lithiated intercalation compounds is1 to 50 μm, and preferably 5 to 20 μm.

In the present invention, a surface-treatment layer comprising theconductive agent and at least one coating-element-containing compound isformed on a surface of the active material. Thecoating-element-containing compound is selected from the groupconsisting of a coating-element-containing hydroxide, acoating-element-containing oxyhydroxide, a coating-element-containingoxycarbonate, a coating-element-containing hydroxycarbonate, and amixture thereof. The coating-element-containing compound has either anamorphous or a crystalline phase.

The coating element is at least one selected from the group consistingof an alkali metal, an alkaline earth metal, a group 13 element, a group14 element, a group 15 element, and a transition metal. The group 13element (according to the new IUPAC agreement) refers to the elementgroup of the Periodic Table including Al. The group 14 element(according to the new IUPAC agreement) refers to the element group ofthe Periodic Table including Si. The group 15 element (according to thenew IUPAC agreement) refers to the element group of the Periodic Tableincluding As. In embodiments of the present invention, the coatingelement includes Mg, Al, Co, K, Na, Ca, Si, Ti, B, As, Zr, Sn, V, Ge,and Ga, or a combination thereof.

The amount of the coating element is 2×10⁻⁵ to 2 wt %, and preferably0.001 to 2 wt % of the active material. When the amount thereof is below0.001 wt %, the effect of coating is not sufficiently high, whereas whenthe amount of the coating element is above 2 wt %, the electrodecapacity is reduced due to the added weight of the coating element.

The conductive agent in the surface treatment layer of the activematerial is selected from a carbon-based conductive agent, agraphite-based conductive agent, a metal-based conductive agent, or ametallic compound-based conductive agent, but it is not limited thereto.

KS6 (produced by the TIMCAL company) is a graphite-based conductiveagent according to an embodiment of the present invention. Thecarbon-based conductive agent may be selected from the group consistingof Super P (produced by the MMM company), ketchen black, denka black,acetylene black, carbon black, thermal black, channel black, andactivated carbon. The metal-based conductive agent may be one or more ofpowdery Pt, Ru, Ni, Ti, La, or Sn. The metallic compound-basedconductive agent may be selected from the group consisting of tin oxide,tin phosphate (SnPO₄), titanium oxide, and perovskite such as LaSrCoO₃,LaSrMnO₃, etc. Any one or more of these, graphite, carbon, metal ormetallic compound-based conductive agents may be used for the coating.

An amount of the conductive agent in the surface-treatment layer is inthe range of 0.5 to 10 wt % of the active material, and preferably inthe range of 1 to 5 wt %. When the amount of the conductive agent isless than 0.5 wt %, the conductivity of the slurry-coated activematerial composite may be reduced to a below-adequate level resulting indeterioration of the electrochemical characteristics such as cycle lifeand high rate capability. When the amount of the conductive agent ismore than 10 wt %, the energy density per weight of the electrode,therefore that of the cell, decreases significantly.

A thickness of the surface-treatment layer is 1 to 300 nm, preferably 1to 100 nm, and more preferably 1 to 50 nm. When the thickness is lessthan 1 nm, the effect of the surface-treatment layer on the batteryperformance is insignificant. If the thickness is more than 300 nm, thethickness is too thick to facilitate the movement of Li⁺ ions throughthe coating layer and to improve the battery performance.

The surface-treated active material is prepared through the preparationmethod comprising adding a conductive agent and a coating-element sourceto a solvent selected from the group consisting of water, organicsolvent, and mixtures thereof to prepare a coating liquid. An activematerial is added to the coating liquid to coat the material. The coatedactive material is dried to form a surface-treatment layer on the activematerial.

Specifically, the coating liquid used in coating the active material isprepared by adding a conductive agent and a coating-element source to asolvent, if necessary by using a reflux technique, to form a solution ora suspension. A “coating liquid” generally refers to a homogeneoussuspension or a solution.

The solvents include an organic solvent or water. The coating-elementsource includes any coating-element or any coating-element-containingcompound that is soluble in the solvent (i.e., an organic solvent orwater). As described above, the coating-element source includes acoating element such as an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a group 15 element, and atransition metal. According to an embodiment of the invention, thecoating element source includes at least one selected from the groupconsisting of Mg, Al, Co, K, Na, Ca, Si, Ti, B, As, Zr, Sn, V, Ge, andGa, or a combination thereof, and preferably, Al or B.

The coating-element source includes a coating-element, acoating-element-containing alkexide such as methoxide, ethoxide, orisopropoxide, a coating-element-containing salt, or acoating-element-containing oxide. Since the solubility of thecoating-element source largely depends upon the type of solvent, onehaving ordinary skill in the art can easily choose a suitablecoating-element source from the group consisting of the coating elementitself and the coating-element-containing alkoxide, thecoating-element-containing salt, or the coating-element-containing oxideby considering the type of solvent. For example, if an organic solventis used as the solvent for the coating liquid, the coating element, thecoating-element-containing alkoxide, the coating-element-containingsalt, or the coating-element-containing oxide may be chosen such that itis dissolved in the organic solvent, using a reflux method if necessary.Alternatively, if water is used as the solvent, either thecoating-element-containing salt or the coating-element-containing oxidemay be used to prepare the coating liquid. For an example of thecoating-element source, tetraethyl orthosilicate may be used as asilicon source, whereas B₂O₃, H₃BO₃, or HB(OH)₂ can be used as a boronsource. HB(OH)₂ is prepared by dissolving B₂O₃ in an organic solvent orwater followed by drying the liquid. When vanadium is used as acoating-element, vanadium oxide (V₂O₅) or a vanadate such as ammoniumvanadate (NH₁(VO)₃) may be examples of the coating element source.

Examples of the organic solvents according to embodiments of theinvention include, but are not limited to, alcohols such as methanol,ethanol, or isopropanol. Other solvents according to embodiments of theinvention include hexane, chloroform, tetrahydrofuran, ether, methylenechloride, and acetone.

An amount of the coating-element source is 0.1 to 50 wt % and preferably5 to 30 wt % of the coating solution according to an embodiment of theinvention. When the amount thereof is below 0.1 wt %, coating is notfully effective, whereas when the amount of the coating-element sourceis more than 50 wt %, the thickness of the resultant surface-treatmentlayer is difficult to control evenly.

The active material powder is surface-treated (coated) using the coatingliquid described above. The coating liquid includes a conductive agentand a coating-element source. The coating process is performed by adipping method according to an embodiment of the invention. The dippingmethod includes dipping the active material in the coating liquid,removing any excess liquid, if necessary, and then drying the dippedactive material. Generally, the dip coating method is used, but it isunderstood other methods can be used.

According to another embodiment of the invention, the coating process isperformed using a single continuous process (hereafter referred to as a“one-shot coating process”). In the one-shot coating process, the mixingof the active material with the coating liquid, the solvent-removing,and the drying take place in a single process vessel. The one-shotcoating process is relatively simple, thereby reducing production costand making a uniform surface-treatment layer on the active materialparticles.

The one-shot coating process will be illustrated in further detail. Fora better understanding, the coating process using the one-shot coatingprocess is schematically shown in FIG. 1B in comparison with the generalcoating process shown in FIG. 1A used for production of the coatedactive material for a lithium ion battery. As shown in FIG. 1A, thegeneral coating technique includes multiple process-operations asdescribed below. In operation 100, an active material is added to thecoating liquid in a mixer. The mixer mixes the active material and thecoating liquid well to coat the active material with the coating liquidin operation 110. In operation 120, the resulting wet active materialwith the coating liquid is transferred to a shallow tray to remove thesolvent by evaporation. In operation 130, the coated active material isdried in the air at an elevated temperature (for example 80 to 100° C.)to produce the active material 140.

On the other hand, the one-shot coating process involves preparing acoating liquid (operation 200), putting the coating liquid and theactive material into a mixer and raising the temperature of the mixerwhile the contents are being agitated during mixing (operation 210).Simultaneously, a purging gas is introduced into the mixing apparatus300, which is schematically shown in FIG. 2. The purging gas facilitatesevaporation of the solvent of the coating liquid and purges impure gasesthat may be present in the mixer. The purging gas may be CO₂, or amoisture-free inert gas such as nitrogen or argon. In this coatingoperation, the active material is coated with the coating liquid in themixer 300, and the solvent of the coating liquid is evaporated andremoved while the process mixture is being continuously stirred. Thesolvent removal process occurs as the temperature is raised.

Therefore, the transfer of the liquid-coated wet active material toanother vessel (a tray) and the separate drying operation (operations120 and 130) in the tray can be combined into a single continuousprocess step in a single vessel. After putting the active material andthe coating liquid in the mixer 300, a premixing process may be furtherperformed for 10 to 30 minutes to obtain a uniform mixture.

The temperature of the mixer 300 is raised to 50 to 100° C., forexample, by circulating hot water through the outside wall of the mixer300 to accelerate evaporation of the solvent such as alcohol or waterusing a heat exchanger 310. The type of mixer 300 is not limited to anyone type as long as it is capable of mixing the active material with thecoating liquid effectively, injecting the purging gas if used, andraising the temperature to a desired value. A representative example ofthe mixer 300 is a planetary mixer.

Subsequent to the wet coating, the coated active material is dried toform the resultant active material 140 or 220 for a battery of thepresent invention. The resultant active material includes thesurface-treatment layer comprising a conductive agent and acoating-element-containing compound such as a coating-element-containinghydroxide, a coating-element-containing oxyhydroxide, acoating-element-containing oxycarbonate, a coating-element-containinghydroxycarbonate, and a mixture thereof.

The drying operations is preferably performed at a temperature in therange of room temperature (i.e., roughly 20° C.) to 200° C., for 1 to 24hours. When the drying temperature is lower than room temperature, thedrying time is unduly prolonged. If the drying temperature is higherthan 200° C., the desired quality of the surface-treatment layer may notbe achieved. When the drying duration is shorter than 1 hour or longerthan 24 hours, the desired quality of the surface-treatment layer maynot be obtained due to the formation of an undesirable crystal structureor morphology.

When using the one-shot coating process in FIG. 1B, a separate dryingoperation 130 of FIG. 1A is not necessary after the coating process 110of FIG. 1A because the drying operation is performed simultaneously withthe coating operation.

During the drying operation 210, the coating element in the coatingliquid on the surface of the active material may react with moisture inthe atmosphere to produce a hydroxide. Thus, the surface-treatment layermay include a new amorphous or crystalline coating-element-containinghydroxide formed on the surface. During the drying operation, thesurface-treatment layer may also produce a coating-element-containingoxyhydroxide, a coating-element-containing oxycarbonate, or acoating-element-containing hydroxidecarbonate due to a partialdehydration of the coating-element-containing hydroxides, reaction withatmospheric carbon dioxide (CO₂) or both.

The final form of the active material is coated with thesurface-treatment layer including coating-element-containing compoundand a conductive agent. The coated active material powder may or may notbe sieved to obtain a powder with a desirable average diameter. Wherethere is no sieving, the same material that is included in thesurface-treatment layer remains in the active material slurry. Theremaining material in the slurry improves the thermal stability of theelectrode.

Since the surface-treatment layer including the conductive agent isformed on the active material, the internal resistance is improvedsignificantly in a battery cell containing the surface-treated activematerial as opposed to a battery cell without the coating. Thus, thecharge and discharge-potential characteristics, including overpotentialon charging and voltage depression on discharging, are improvedsignificantly in the battery with the coated active material. It isthereby anticipated to enhance the power capability of the battery celland also provide good cycle-life of the battery cell.

In accordance with another preferred embodiment, the present inventionalso provides a high density active material composite for the electrodefor the battery. The conventional preparation process of an electrodefor a lithium ion battery is compared with an electrode preparationprocess of the present invention in FIGS. 3A and 3B.

As shown in FIG. 3A, an electrode of the prior art is generallyfabricated by casting (coating) a slurry of an active material on acurrent collector (operations 400 to 420) and then compressing it(operation 430). The slurry is prepared by dry mixing an active materialand a conductive agent followed by adding the mixture to abinder-containing solution (operations 400 and 410). The conductiveagent has a large surface area over 2500 m²/g and a high volumeresulting in a low density active material composite on the electrode.The resultant electrode shows a reduced energy density.

In the present invention, however, the electrode is fabricated bysuspending the conductive agent-coated active material in abinder-containing solution to prepare the active material slurryfollowed by casting the slurry on a current collector and compressing itas shown in operations 500 to 540 of FIG. 3B. The binder-containingsolution is prepared by adding a binder material to any conventionalsolvent used for the conventional active material slurry, such asN-methyl pyrrolidone (NMP). The contents of the active material, bindermaterial, and solvent should be appropriate to provide a suitableviscosity so that the active material slurry can be readily cast on thecurrent collector.

In the embodiment of the present invention, a reduced amount of theconductive agent can provide the active material composite with acomparable conductivity of the composition prepared by thestate-of-the-art process containing an excess amount of the conductiveagent. This improvement is mainly due to the intimate contact of theconductive agent with the active material particles by the nature of thecoating.

The reason of the improvement described above is illustratedschematically in FIGS. 4A and 4B. In the active material composite bythe state-of-the-art process as shown schematically in FIG. 4A, theactive material particles 1 and the conductive agent particles 2 arerather loosely distributed so that the overall density of the activematerial composite is not as high as the active material composite ofthe present invention as shown in FIG. 4B. As shown in FIG. 4B, theactive material particles 10 and the conductive agent particles 30 aredistributed rather compactly in the composite. The conductive agentparticles 30 are held in position tightly by thecoating-element-containing compound 20. As the result of such compactparticle distributions, the present invention provides a battery with ahigh energy density (energy per unit volume) of the electrode andtherefore provides a high energy density battery.

In addition, in FIG. 4A, the active material particles 1 and theconductive agent particles 2 are not intimately in contact with eachother. Thus, there is a reduced efficiency in conductivity by theconductive agent 2 as compared with the active material composite of thepresent invention as shown in FIG. 4B. As shown in FIG. 4B, the activematerial particles 10 and the conductive agent particles 30 are inintimate contact with each other. As the result of such intimatecontacts, a smaller weight portion of the conductive agent is needed inthe active material composite of the present invention as opposed to theweight needed for the state-of-the-art process to provide the activematerial composite with an equivalent conductivity. Therefore, thespecific energy (energy per unit weight) of the electrode by the presentinvention is improved over that of the state-of-the-art process.

Another advantage of the present invention is a reduction in theprocessing time of the electrode preparation. This reduction saves asignificant production cost by eliminating a powder mixing processoperation from the state-of-the-art process as shown in FIGS. 3A and 3B.

Still another advantage of the present invention is that the chemicaland/or electrochemical stability of the electrolyte/active materialinterface is improved substantially by the presence of thecoating-element-containing compound 20 as shown in FIG. 4B. The improvedstability, in turn, improves the cycle life of the electrode as well asreduces undesirable gas generation in the cell which builds up theinternal pressure of the cell. These improvements in cycle life as wellas in the gas generation are due to a reduction in irreversibleoxidation reactions of the electrolyte at the surface of the chargedactive material by the presence of the coating layer.

Other important characteristics of a commercially viable battery are itssafety characteristics. The battery is required to be safe in using inoccasional, abusive conditions. These abuses are due to mechanical,electrical and thermal abuses. Examples of mechanical abuse testsinclude a nail penetration test, in which a metallic nail penetratesinto a fully charged cell, a mechanical crush test of the charged cell,and other similar tests. Examples of electrical abuse tests include anovercharge test at various charge rates, and over-discharge test, andother similar tests. These safety characteristics of the battery areclosely related to an exothermic reaction of the charged active materialand the electrolyte.

The rate of such an exothermic reaction is usually measured using adifferential scanning calorimetry (DSC). For example, a coin-typehalf-cell with a LiCoO₂ active material is charged to a pre-determinedpotential to convert LiCoO₂ to Li_(1-x)CoO₂, where x is close to 0.5. Asmall piece of the charged electrode from the cell is subjected to a DSCtest to evaluate the thermal stability of the charged active material.Namely, the thermal stability of the active material is evaluated by anexothermic starting temperature and a quantity of heat evolution fromthe DSC measurement. Because the Li_(1-x)CoO₂ active material is anunstable compound, oxygen is bonded to the metal (Co—O) until a certainvalue of temperature (exothermic peak), at which it decomposes as thetemperature increases and gaseous oxygen is released. The releasedoxygen may react with the electrolyte in a cell to cause a pressurebuild-up in the cell and, in the worst case, leads to an explosion.Accordingly, the exothermic peak temperature and the quantity of heatevolved are closely related to the safety.

The active material of the present invention has a relatively highexothermic starting temperature and a small quantity of heat evolutionduring the exothermic reaction. Thus, the active material of the presentinvention exhibits superior thermal stability as compared to aconventional active material.

The present invention is further explained in more detail with referenceto the following specific examples. These specific examples, however,should not in any sense be interpreted as limiting the scope of thepresent invention.

Comparative Example 1

LiCoO₂ powder for a positive active material (average particle diameter:10 μm) and Super P for a conductive agent were pre-mixed. The resultantmixture was added to a polyvinylidene fluoride binder solution. Theweight ratio of LiCoO₂ powder, Super P, and polyvinylidene fluoride was94:3:3. The mixture was mixed in an appropriate amount of N-methylpyrrolidone (NMP) solvent thoroughly for 2 hours and aged for 1 hour, toprepare a positive active material slurry. The positive active materialslurry was cast onto an Al foil to a thickness of about 100 μm, followedby drying and compressing the coated Al foil. The resultantslurry-coated Al foil was cut into a disk having a diameter of 1.6 cm toprepare a positive electrode.

Using the positive electrode and a lithium counter electrode, acoin-type half-cell was fabricated in an Ar-purged glove box. For theelectrolyte, a 1 M LiPF₆ solution in ethylene carbonate and dimethylcarbonate (1:1 volume ratio) was used.

Comparative Example 2

A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except that LiMn₂O₄ (average particle diameter:15 μm) powder was used instead of the LiCoO₂ powder.

Comparative Example 3

A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except that LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂(average particle diameter: 10 μm) powder was used instead of the LiCoO₂powder.

Comparative Example 4

A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except thatLiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂ (average particle diameter: 10μm) powder was used instead of the LiCoO₂ powder.

Comparative Example 5

A coin-type half-cell was fabricated by the same procedure as inComparative Example 1, except that the LiCoO₂ powder, Super P, andpolyvinylidene fluoride were mixed in the weight ratio of 96:2:2.

Reference Example 1

A 1 wt % Al-isopropoxide coating suspension was prepared by adding 2 gof Al-isopropoxide powder to 198 g of ethanol. 1 kg of the LiCoO₂ powder(average particle diameter: 10 μm) was added to 200 g of theAl-isopropoxide coating suspension, followed by mixing it thoroughly forabout 10 minutes in a mixer with a water-jacketed heat exchanger to coatthe surface of the LiCoO₂ powder with the suspension. Purging nitrogengas was injected into the mixer while the mixer chamber temperature waskept at 60° C. by circulating hot water through the heat exchanger. Themixture was stirred continuously for an additional 1 hour while drynitrogen gas was flowing continuously to remove the ethanol byevaporation and to thereby obtain a coated LiCoO₂ active material powderwith a layer of AlO(OH).

The coated LiCoO₂ powder and the Super P for a conductive agent werepre-mixed, and the resultant mixture was added to a polyvinylidenefluoride binder solution. The weight ratio of the coated LiCoO₂ powder,the Super P, and polyvinylidene fluoride was 94:3:3. The mixture in anappropriate amount of the N-methyl pyrrolidone (NMP) solvent was mixedthoroughly, to prepare a positive active material slurry. The positiveactive material slurry was cast onto an Al foil in a thickness of about100 μm followed by drying and compressing the coated Al foil. Theresultant slurry-coated Al foil was cut into a disk having a diameter of1.6 cm to prepare a positive electrode.

Using the positive electrode and the lithium counter electrode, acoin-type half-cell was fabricated in an Ar-purged glove box. For theelectrolyte, a 1 M LiPF₆ solution in ethylene carbonate and dimethylcarbonate (1:1 volume ratio) was used.

Reference Example 2

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that a 5 wt % Al-isopropoxide coatingsuspension was used instead of the 1 wt % Al-isopropoxide coatingsuspension.

Reference Example 3

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that a 10 wt % Al-isopropoxide coatingsuspension was used instead of the 1 wt % Al-isopropoxide coatingsuspension.

Reference Example 4

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that the coated LiCoO₂ powder, the Super P,and the polyvinylidene fluoride were mixed in the weight ratio of96:2:2.

Reference Example 5

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that the coated LiCoO₂ powder, the Super P,and the polyvinylidene fluoride were mixed in the weight ratio of96:1:3.

Reference Example 6

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiMn₂O₄ (average particle diameter: 15μm) powder was used instead of the LiCoO₂ powder.

Reference Example 7

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiMn₂O₄ (average particle diameter: 15μm) powder was used instead of the LiCoO₂ powder, and the coated LiMn₂O₄powder, the Super P, and the polyvinylidene fluoride were mixed in theweight ratio of 96:1:3.

Reference Example 8

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ (averageparticle diameter: 10 μm) powder was used instead of the LiCoO₂ powder.

Reference Example 9

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ (averageparticle diameter: 10 μm) powder was used instead of the LiCoO₂ powder,and the coated LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ powder, the Super P, andthe polyvinylidene fluoride were mixed in the weight ratio of 96:1:3.

Reference Example 10

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂(average particle diameter: 10 μm) powder was used instead of the LiCoO₂powder.

Reference Example 11

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that LiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂(average particle diameter: 10 μm) powder was used instead of the LiCoO₂powder, and the coated LiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂ powder,the Super P and the polyvinylidene fluoride were mixed in the weightratio of 96:1:3.

Reference Example 12

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that a 5 wt % boron ethoxide suspension,which was prepared by adding 10 g of B₂O₃ powder to 190 g of the ethanolwas used, and the resulting LiCoO₂ had a coating layer of HB(OH)₂.

Reference Example 13

A coin-type half-cell was fabricated by the same procedure as inReference Example 1, except that a 10 wt % boron ethoxide suspension,which was prepared by adding 20 g of B₂O₃ powder to 180 g of the ethanolwas used, and the resulting LiCoO₂ had a coating layer of HB(OH)₂, andthe coated LiCoO₂ powder, the Super P, and the polyvinylidene fluoridewere mixed in the weight ratio of 96:1:3.

Example 1

4 g of Al-isopropoxide powder was added to 396 g of ethanol to prepare400 g of Al-isopropoxide suspension. 63.8 g of the Super P and 800 g ofthe ethanol were added to the Al-isopropoxide suspension to prepare acoating liquid. Using the coating liquid, the LiCoO₂ powder (averageparticle diameter: 10 μm) having a surface-treatment layer comprising acoating layer of the AlO(OH) and the Super P on the surface thereof wasproduced by the same procedure as in coating process of ReferenceExample 1. The amount of the Al was 0.2 wt % of the total activematerial weight and the amount of the Super P was 3 wt % of the totalactive material weight.

The coated LiCoO₂ powder was suspended in a solution of polyvinylidenefluoride binder in N-methyl pyrrolidone solvent to prepare a positiveactive material slurry. The slurry contained the coated LiCoO₂ powderand the polyvinylidene fluoride in the weight ratio of 97:3 with anappropriate amount of the N-methyl pyrrolidone solvent. The positiveactive material slurry was cast onto an Al foil in a thickness of about100 μm followed by drying and compressing the coated Al foil. Theresultant slurry-coated Al foil was cut into a disk having a diameter of1.6 cm to prepare a positive electrode.

Using the positive electrode and a lithium metal counter electrode, acoin-type half-cell was fabricated in an Ar-purged glove box. For theelectrolyte, a 1 M LiPF₆ solution in a mixed solvent of ethylenecarbonate (EC) and dimethyl carbonate (DMC) in the volume ratio of 1:1was used.

Example 2

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 63.8 g of the Super P and800 g of ethanol to the suspension. The amount of the Al in the coatedLiCoO₂ powder was 1 wt % of the total active material weight, and theamount of the Super P was 3 wt % of the total active material weight.

Example 3

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 41.6 g of the Super P and800 g of the ethanol to the suspension. The positive active materialslurry contained the coated LiCoO₂ and the polyvinylidene fluoride in aweight ratio of 98:2 with an appropriate amount of the N-methylpyrrolidone solvent. The amount of Al in the coated LiCoO₂ powder was 1wt % of the total active material weight, and the amount of the Super Pwas 2 wt % of the total active material weight.

Example 4

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 40 g of theAl-isopropoxide powder to 360 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 63.8 g of the Super P and800 g of the ethanol to the suspension. The amount of the Al in thecoated LiCoO₂ powder was 2 wt % of the total active material weight, andthe amount of the Super P was 3 wt % of the total active materialweight.

Example 5

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 20.8 g of the Super P and800 g of the ethanol to the suspension. The amount of the Al in thecoated LiCoO₂ powder was 1 wt % of the total active material weight, andthe amount of the Super P was 1 wt % of the total active materialweight.

Example 6

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 63.8 g of the Super P and800 g of the ethanol to the suspension, and a LiMn₂O₄ (average particlediameter: 15 μm) powder was used instead of the LiCoO₂ powder. Theamount of the Al in the coated LiMn₂O₄ was 1 wt % of the total activematerial weight, and the amount of the Super P was 3 wt % of the totalactive material weight.

Example 7

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 20.8 g of the Super P and800 g of the ethanol to the suspension, and a LiMn₂O₄ (average particlediameter: 15 μm) powder was used instead of the LiCoO₂ powder. Theamount of the Al in the coated LiMn₂O₄ was 1 wt % of the total activematerial weight, and the amount of the Super P was 1 wt % of the totalactive material weight.

Example 8

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 63.8 g of the Super P and800 g of the ethanol to the suspension, and aLiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ (average particle diameter: 10 μm) powderwas used instead of the LiCoO₂ powder. The amount of the Al in thecoated LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂, was 1 wt % of the total activematerial weight, and the amount of the Super P was 3 wt % of the totalactive material weight.

Example 9

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 20.8 g of the Super P and800 g of the ethanol to the suspension, and aLiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ (average particle diameter: 10 μm) powderwas used instead of the LiCoO₂ powder. The amount of the Al in thecoated LiNi_(0.9)Sr_(0.002)Co_(0.1)O₂ was 1 wt % of the total activematerial weight, and the amount of the Super P was 1 wt % of the totalactive material weight.

Example 10

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 63.8 g of the Super P and800 g of the ethanol to the suspension, and aLiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂ (average particle diameter: 10μm) powder was used instead of the LiCoO₂ powder the amount of the Al inthe coated LiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂ was 1 wt % of thetotal active material weight, and the amount of the Super P was 3 wt %of the total active material weight.

Example 11

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl-isopropoxide powder to 380 g of the ethanol to prepare 400 g of theAl-isopropoxide suspension, followed by adding 20.8 g of the Super P and800 g of ethanol to the suspension, and aLiNi_(0.66)Mn_(0.25)Al_(0.05)CO_(0.1)O₂ (average particle diameter: 10μm) was used instead of the LiCoO₂ powder. The amount of the Al in thecoated LiNi_(0.66)Mn_(0.25)Al_(0.05)Co_(0.1)O₂ was 1 wt % of the totalactive material weight, and the amount of the Super P was 1 wt % of thetotal active material weight.

Example 12

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of B₂O₃powder to 380 g of the ethanol to prepare 400 g of boron ethoxidesuspension, followed by adding 63.8 g of the Super P and 800 g of theethanol to the suspension, and the resulting LiCoO₂ had a coating layerof HB(OH)₂. The amount of the B In the coated LiCoO₂ was 1 wt % of thetotal active material weight, and the amount of the Super P was 3 wt %of the total active material weight.

Example 13

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 40 g of theB₂O₃ powder to 360 g of the ethanol to prepare 400 g of boron ethoxidesuspension, followed by adding 20.8 g of the Super P and 800 g of theethanol to the suspension, and the resulting LiCoO₂ had a coating layerof HB(OH)₂. The amount of the B in the coated LiCoO₂ was 2 wt % of thetotal active material weight, and the amount of the Super P was 1 wt %of the total active material weight.

Example 14

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 4 g of Al(NO₃)₃powder to 396 g of water to prepare 400 g of Al(NO₃)₃ suspension,followed by adding 63.8 g of the Super P and 800 g of the water to thesuspension. The amount of the Al in the coated LiCoO₂ was 0.2 wt % ofthe total active material weight, and the amount of the Super P was 3 wt% of the total active material weight.

Example 15

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 20 g of theAl(NO₃)₃ powder to 380 g of the water to prepare 400 g of the Al(NO₃)₃suspension, followed by adding 63.8 g of the Super P and 800 g of thewater to the suspension. The amount of the Al in the coated LiCoO₂ was 1wt % of the total active material weight, and the amount of the Super Pwas 3 wt % of the total active material weight.

Example 16

A coin-type half-cell was fabricated by the same procedure as in Example1, except that the coating liquid was prepared by adding 40 g ofAl(NO₃)₃ powder to 360 g of water to prepare 400 g of Al(NO₃)₃suspension, followed by adding 63.8 g of Super P and 800 g of water tothe suspension. The amount of Al in the coated LiCoO₂ was 2 wt % of thetotal active material weight, and the amount of Super P was 3 wt % ofthe total active material weight.

Example 17

A coin-type half-cell was fabricated by the same procedure as in Example1, except that artificial graphite powder was used instead of the LiCoO₂powder.

Example 18

A coin-type half-cell was fabricated by the same procedure as in Example1, except that natural graphite powder was used instead of the LiCoO₂powder.

Example 19

A coin-type half-cell was fabricated by the same procedure as in Example1, except that SnO₂ powder was used instead of the LiCoO₂ powder.

Example 20

A coin-type half-cell was fabricated by the same procedure as in Example1, except that powdery silicon (Si) active material was used instead ofthe LiCoO₂ powder.

The time taken to produce a slurry used during the fabrication ofelectrodes for cells using 2 kg of the active materials according toComparative Example 1, Reference Example 5, and Example 5 are as shownin Table 1.

TABLE 1 Mixing time the Premixing mixture of active time of activematerial/conductive material and material and Aging Sum conductivebinder-containing time (in agent (in hours) solution (in hours) (inhours) hours) Comp. Ex. 1 1 2 1 4 Ref. Ex. 5 1 2 1 4 Ex. 5 — 2 — 2

For the active material powder samples according to Comparative Example1 and Reference Example 5, it took 4 hours each to prepare the slurryfor even mixing of the active material and conductive agent. Conversely,for the active material which is already coated with the conductiveagent in Example 5, a uniform slurry was achieved within 2 hours. Asshown in Table 1, the coated active material according to the presentinvention may reduce the slurry preparation time in fabrication of theelectrode by as much as 50%. The productivity in fabrication ofbatteries is expected to be improved even more in a large-scaleproduction.

The LiCoO₂ active material prepared according to Example 1 of thepresent invention has a coating (surface-treatment) layer comprising acoating-element-containing compound and a conductive agent, and theSuper P on the surface of the active material. SEM photographs of theuncoated (Comparative Example 1) and the coated LiCoO₂ (Example 1) inthe positive electrode samples are shown in FIGS. 5A and 5B,respectively. In FIG. 5A, the Super P is non-uniformly distributedamongst the LiCoO₂ powder. On the other hand, in FIG. 5B, the Super P isuniformly distributed on the surface of the LiCoO₂ powder, resulting ina more uniform distribution of the Super P in the electrode than oneprepared using the uncoated LiCoO₂ powder and the Super P.

In order to evaluate the capacity characteristics of the coin-typehalf-cells of Comparative Examples, Reference Examples, and Examples atvarious rates, the cells were charged-discharged in the voltage range of4.3 to 2.75 V. Results of the charge-discharge characteristics atvarious charge and discharge rates (0.2C, 0.5C, 1C) of Reference Example5 and Example 5 are shown in FIGS. 6A and 6B respectively. The LiCoO₂powder of Example 5, with a coating layer of AlO(OH) and Super Pconductive agent, are superior in charge and discharge characteristicsto the LiCoO₂ powder of Reference Example 5 with a coating layer ofAlO(OH) only. When the charge and discharge rate is increased from a lowrate (0.2C) to a high rate (1.0C), the superiority is especiallypronounced. In addition, the discharge characteristics of ComparativeExample 5, Reference Example 4, and Example 3 at a high rate (1C)indicate that the discharge characteristics of the cell of Example 3 issignificantly better than those of Comparative Example 5 and ReferenceExample 4 as shown in FIG. 7.

FIG. 8. shows a cross-sectional view of a prismatic Li-ion cellaccording to an embodiment of the invention. The prismatic Li-ion cell 3is fabricated by inserting an electrode assembly 4 including a positiveelectrode 5, a negative electrode 6, and a separator 7 between thepositive and negative electrodes into a battery case 8. An electrolyteis injected therein and the upper part of the case 8 is sealed with acap plate 11 and a gasket 12. It is understood that other types ofbatteries can be constructed using the coated active material of thepresent invention.

The positive electrode 5 of the sample Li-ion cells was prepared by thesame electrode preparation method as described in Example 1 orComparative Example 1. A negative electrode 6 of the sample Li-ion cellswas prepared by casting a slurry including the negative active materialon a Cu foil. The slurry containing negative active material wasprepared by mixing an artificial graphite powder as the negative activematerial and a polyvinylidene fluoride as a binder material in theweight ratio of 92:8 in an NMP. For the electrolyte, a 1M LiPF₆ solutionin a mixed solvent of ethylene carbonate and dimethyl carbonate in thevolume ratio of 1:1 was used.

Prismatic Li-ion cells of a nominal capacity of 670 mAh comprising thepositive active materials prepared in Examples 3 and Comparative Example5 were charged-discharged at 1C rate in the voltage range of 4.2 to 2.75V in order to evaluate their cycle life characteristics. The results ofcycle life tests of these cells are shown in FIG. 9. As indicated inFIG. 9, the positive active material of Example 3 has superiorcycle-life characteristics to those of Comparative Example 5 during 300charge-discharge cycles.

FIG. 10 shows discharge characteristics at a low temperature of −20° C.of the 670-mAh prismatic Li-ion cells fabricated as described above, butusing the positive active materials of Reference Example 5 and Example5. As shown in FIG. 10, the cell containing the active material fromExample 5 shows an improved capacity by about 80 mAh over the cellcontaining the active material from Reference Example 5.

In order to evaluate thermal stability of the positive active materialprepared according to Example 5 of the present invention and ComparativeExample 5, DSC analyses were performed. Specifically, the coin-type halfcell fabricated in Example 5 and Comparative Example 5 were chargedusing a voltage cut-off at 4.3V. About 10-mg portions of the positiveactive material from charged electrodes from each cell were removed forthe DSC tests. DSC analyses were carried out in a sealed aluminum canusing scanning temperatures from 100 to 300° C. at the rate of 3° C./minusing a 910 DSC (produced by TA INSTRUMENTS Inc.) instrument. Theresults are shown in FIG. 11.

As shown in the FIG. 11, Comparative Example 5 (uncoated LiCoO₂) showeda large exothermic peak in the temperature range of 190 to 220° C. as aresult of O₂ release from the breakage of Co—O bonds of chargedLi_(1-x)CoO₂ followed by an exothermic reaction of the oxygen with theelectrolyte. This phenomenon can cause of safety problems for a Li-ioncell. However, in the case of Example 5, the exothermic peak in the DSCwas reduced to a negligible size, strongly suggesting that the thermalstability and, therefore, the safety of the batteries using the activematerial of Example 5 will be much better than that of ComparativeExample 5.

Five 670-mAh prismatic Li-ion cells comprising the positive activematerial from Comparative Example 1 and the positive active materialsprepared in Reference Example 5 and Example 5 were tested for anovercharge safety test at 1C rate. The sample Li-ion cells werefabricated as described above. The overcharge test was carried out byobserving changes of the sample cells that were overcharged at 1C ratefor 150% of the nominal capacity. The results are presented in Table 2.

TABLE 2 Comp. Ex. 1 Ref. Ex. 5 Ex. 5 Overcharge test 3L2, 2L1 5L1 5L0 at1C rate Note: The number preceding “L” indicates the number of testedcells and the number following “L” indicates level of safety.

The results of the safety test were rated as follows: L0: no leakage,L1: a visual sign of leakage, and L2: an electrolyte flame withoutthermal runaway.

As shown in Table 2, the positive active material of Example 5 showsmuch improved safety over those of Reference Example 5 and ComparativeExample 1.

In order to see the adhesion strength of the active material compositeto the current collector for the electrode samples, a peeling test wasperformed with the electrodes from Comparative Example 1, ReferenceExample 2, and Example 2, respectively, before and after compression.The results are shown in Table 3.

TABLE 3 Comp. Ex. 1 Ref. Ex. 2 Ex. 2 gf/cm gf/cm gf/cm Beforecompression UM UM 4-6 After compression 1-2 2-4 12-18 Note: UM in Table3 indicates “incapability of measurement since the adhesion strength ofthe cast slurry film to the current collector is too weak to measurequantitatively.”

As shown in Table 3, the electrode prepared using the positive activematerial of Example 2 shows an adhesion strength at least four times ashigh as those prepared using the positive active material of ComparativeExample 1 and Reference Example 2. As the adhesion strength is improvedbetween the active material and current collector, the detachment of theactive material from the current collector during charge-dischargecycles will be prevented, resulting in a reduced change in internalresistance of the cell with cycling.

Since the active material coated with a coating-element-containingcompound and a conductive agent gives an improved adhesion strength ofthe active material composite to the current collector, a reduced amountof the binder material may be used to achieve an adequate adhesionstrength for a Li-ion cell. Therefore, an increased proportion of theactive material may be used in the cell by the present invention, whichimproves energy density and specific energy of the cell. In addition,through proportional reduction of the binder quantity, a saving of thematerial cost may also be realized by the use of the present invention.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the accompanying claims and equivalents thereof.

1. A method of preparing an active material for a battery comprising:adding a conductive agent selected from a carbon-based conductive agent,a graphite-based conductive agent, a metal-based conductive agent, or ametallic compound-based conductive agent and a coating-element source toa solvent selected from the group consisting of water, organic solvent,and a mixture thereof to prepare a coating liquid; adding the activematerial to the coating liquid to coat the active material; and dryingthe coated active material to form a surface-treatment layer comprisingthe conductive agent and at least one coating-element-containingcompound selected from the group consisting of acoating-element-containing hydroxide, a coating-element-containingoxyhydroxide, a coating-element-containing oxycarbonate, acoating-element-containing hydroxycarbonate, and a mixture thereof,wherein the active material is selected from the group consisting of alithium-containing alloy, a compound that reversibly forms alithium-containing compound by a reaction with lithium ions, a materialthat reversibly intercalates/deintercalates the lithium ions and alithiated intercalation compound.
 2. The method of claim 1, wherein theactive material is the lithiated intercalation compound that is selectedfrom the group consisting of a lithium-containing metal oxide, alithium-containing chalcogenide compound, and a carbon-based material.3. The method of claim 1, wherein the active material is the lithiatedintercalation compound that is at least one selected from the groupconsisting of a lithium compound with the following formulas (1) to(13):Li_(x)Mn_(1-y)M_(y)A₂  (1)Li_(x)Mn_(1-y)M_(y)O_(2-z)X_(z)  (2)Li_(x)Mn₂O_(4-z)X_(z)  (3)Li_(x)Mn_(2-y)M_(y)A₄  (4)Li_(x)Co_(1-y)M_(y)A₂  (5)Li_(x)Co_(1-y)M_(y)O_(2-z)X_(z)  (6)Li_(x)Ni_(1-y)M_(y)A₂  (7)Li_(x)Ni_(1-y)M_(y)O_(2-z)X_(z)  (8)Li_(x)Ni_(1-y)Co_(y)O_(2-z)X_(z)  (9)Li_(x)Ni_(1-y-z)Co_(y)M_(z)A_(a)  (10)Li_(x)Ni_(1-y-z)Co_(y)M_(z)O_(2-a)X_(a)  (11)Li_(x)Ni_(1-y-x)Mn_(y)M_(z)A_(a)  (12)Li_(x)Ni_(1-y-z)Mn_(y)M_(z)O_(2-a)X_(a)  (13) wherein 0.95≦x≦1.1;0≦y≦0.5; 0≦z≦0.5; 0≦a≦2; M is at least one element selected from thegroup consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and rare earthelements; A is at least one element selected from the group consistingof O, F, S, and P; and X is at least one element selected from the groupconsisting of F, S, and P.
 4. The method of claim 1, wherein thecoating-element source is soluble in an organic solvent or water.
 5. Themethod of claim 4, wherein the coating-element source is at least oneselected from the group consisting of an alkali metal, an alkaline earthmetal, a group 13 element of the Periodic Table, a group 14 element ofthe Periodic Table, a group 15 element of the Periodic Table, and atransition metal.
 6. The method of claim 5, wherein the coating-elementsource is at least one selected from the group consisting of Mg, Al, Co,K, Na, Ca, Si, Ti, B, As, Zr, Sn, V, Ge, Ga, and a combination thereof.7. The method of claim 1, wherein an amount of the coating-elementsource is 0.1 to 50% by weight of the coating liquid.
 8. The method ofclaim 7, wherein the amount of the coating-element source is 5 to 30% byweight of the coating liquid.
 9. The method of claim 1, wherein saidadding the active material to the coating liquid to coat the activematerial and said drying the coated active material comprises mixing amixture of the active material with the coating liquid while heating themixture of the active material and the coating liquid by increasing atemperature of the mixture.
 10. The method of claim 9, wherein saidmixing of the mixture further comprises mixing under one of a vacuumcondition and a condition of injecting purging gas, while increasing thetemperature.
 11. The method of claim 1, further comprising sieving thedried coated active material through a sieve.
 12. A method of preparingan active material for a battery comprising: adding a conductive agentand a coating-element source to a solvent selected from the groupconsisting of water, organic solvent, and a mixture thereof to prepare acoating liquid; adding the active material to the coating liquid to coatthe active material; and drying the coated active material to form asurface-treatment layer comprising the conductive agent and at least onecoating-element-containing compound selected from the group consistingof a coating-element-containing hydroxide, a coating-element-containingoxyhydroxide, a coating-element-containing oxycarbonate, acoating-element-containing hydroxycarbonate, and a mixture thereof,wherein the active material is a lithiated intercalation compound withthe following formula:Li_(x)Co_(1-y)M_(y)A₂  (5) wherein 0.95≦x≦1.1; 0≦y≦0.5; 0≦z≦0.5; 0≦a≦2;M is at least one element selected from the group consisting of Al, Ni,Co, Mn, Cr, Fe, Mg, Sr, V, and rare earth elements; A is at least oneelement selected from the group consisting of O, F, S, and P; and X isat least one element selected from the group consisting of F, S, and P.